During the coming year this work will be continued with emphasis on evaluation of possible markers differentiating the eastern and western strain of WEE virus; on comparison of biological characteristics of eastern and western strains of the virus (No. 72666 and Sp.No. 73895) in Cs. melanura and several species of Aedes mosquitoes that are present in the eastern U.S. and of epidemiologic significance as possible vectors of WEE to man and horses. During the first year of this study the susceptibility of Cs. melanura and seven other species to infections with WEE was studied, but in those virus-vector experiments the mosquitoes used were of primary importance in the enzootic cycle of the virus and only one Aedes species (Ae. canadensis) was included because of lack of suitable experimental supply of other Aedes species. In the next year of study particular effort will be made to obtain Aedes vexans and Aedes triseriatis of local origin for this purpose.